It is common to transmit electrical signals using a back wiring board (BWB) that is housed in a communication device or an information processing apparatus and on which a plug-in unit(s) is mounted. The transmission speed of recent communication devices has increased due to increased transmission capacity, and thus a technique such as pre-emphasis and line equalization is used to ensure transmission quality. The settings for pre-emphasis and line equalization are adjusted to be favorable for each path to achieve a favorable transmission quality.
The communication device includes multiple paths from a transmission unit to a reception unit via the back wiring board. In a high-speed path, the transmission unit includes a pre-emphasis unit and the reception unit includes a line equalization unit. The reception unit also includes a signal monitoring unit that monitors the state (waveform) of a received signal. The communication device adjusts the settings of the pre-emphasis unit and the line equalization unit, based on the monitoring results such that the state of the received signal becomes favorable.
Such a high-density and high-speed communication device has a problem of crosstalk that occurs between paths. Generally, crosstalk occurs between paths (parallel wiring patterns) close to each other and at connectors of the back wiring board and the transmission unit or the reception unit. Thus, technology for checking crosstalk noise by a program, based on the direction of signal transmission of the path, the signal (drive circuit) timing, etc. has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 2000-67104).
In such a high-density and high-speed communication device, the waveform of the received signal is distorted due to the crosstalk between paths. Thus, if the emphasis/equalization settings are adjusted based on the state of the received signal at the reception unit (in this case, the distorted waveform), the entire waveform is distorted and the transmission characteristics expected from the path specifications cannot be achieved.
FIG. 10 is a diagram for explaining the effect of crosstalk on the adjustment of pre-emphasis. A transmission waveform A represents a pulsed transmission waveform during the adjustment of pre-emphasis. The corresponding reception waveform A represents a reception waveform during the adjustment of pre-emphasis, on which crosstalk XT is imposed thereby making the apparent amplitude of the rising edge a larger than the actual amplitude. The reception unit makes a determination concerning emphasis solely based on the reception waveform A affected by the crosstalk XT, and erroneously determines that the reception waveform A is good and pre-emphasis control at the transmission unit is not necessary.
Consequently, the transmission unit transmits a signal having a transmission waveform B without pre-emphasis (the same as the transmission waveform A) even when crosstalk XT occurs, and the reception unit detects a reception waveform B for which the transmission characteristics cannot be enhanced due to the shallow rising edge b and the small eye. On the other hand, even when there is no crosstalk, the eye can be made larger by determining at the emphasis determination to perform pre-emphasis. The transmission waveform and the reception waveform in this case are represented by a transmission waveform C and a reception waveform C. It is preferable for the reception unit to receive a good reception waveform C having a sharp rising edge c, which is impossible in the conventional technology.
As described above, to eliminate the effect of crosstalk by adjusting the emphasis/equalization settings, it is necessary to adjust the settings appropriately based on the state of crosstalk. However, as described above, emphasis/equalization settings simply based on the reception waveform cannot achieve an appropriate reception waveform, thereby preventing the enhancement of transmission characteristics.